Microlithography is used for the production of microstructured components such as, for example, integrated circuits or liquid crystal displays (LCDs). The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (also referred to as a reticle) illuminated by the illumination system is projected by means of the projection objective on to a substrate (for example, a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate. Mirrors are used as optical components for the imaging process in projection objectives designed for the extreme ultra-violet (EUV) range, that is to say at wavelengths of, for example, about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials.
In the lithography process unwanted defects on the mask have a particularly detrimental effect as they can be reproduced with each illumination step and there is thus the danger that in the worst-case scenario the entire production of semiconductor components is unusable. It is therefore a matter of great significance for the mask to be checked for adequate imaging capability before use thereof in mass production. In that respect the changeover from vacuum ultra-violet (VUV) systems to EUV systems is linked not only to changes in the materials and process steps used, but in particular also to a higher level of sensitivity (typically by four times) of the reflectively designed EUV mask, in relation to topological defects, in comparison with conventional VUV masks.
In that respect the problem which inter alia arises in practice is that, depending on the respective form of the defects and the position thereof relative to the structure to be imaged, in the mask, deviations which are difficult to predict occur in the imaging process. Therefore direct analysis of the imaging effect of possible defect positions is desirable for minimizing the mask defects and for implementing successful mask repair. There is thus a need for the mask to be quickly and easily tested, more specifically as far as possible under the same conditions as occur really in the projection exposure apparatus. It is to be observed in that respect that different degrees of coherence of the light, different illumination settings and greater and greater numerical apertures involving values of NA=0.35 and above are set in the illumination system of current EUV systems, which in practice represents a demanding challenge in terms of emulation or reproduction of the imaging procedure of the projection exposure apparatus in mask inspection.